The invention relates to semiconductor device comprising a distributed-type monolithic integrated circuit on a substrate, operating in the high frequency and/or microwave range, this distributed circuit comprising a plurality of coupled stages having each at least a transistor with electrodes, at least one of these electrodes called, first electrode, being AC connected to ground.
The invention finds its application in realising monolithic microwave integrated circuits (MMICs) to be used in the field of telecommunications.
A distributed amplifier is already known from U.S. Pat. No. 4,853,649. This document describes an amplifier comprising a plurality of coupled field effect transistor elements whose gates are connected to a gate line and whose drains are connected to a drain line, this gate line and drain line being formed by inductors. On the other hand, the sources of the transistors in this amplifier have a self-bias circuit for regulating the DC source potential to a given value.
This source self-bias circuit which is connected to each individual transistor of the distributed amplifier has a capacitor and a resistor arranged in parallel between each source and ground.
The capacitor is used for connecting the source to ground in the microwave range. Thus it has to have a sufficiently high value to realise a microwave short-circuit, for example, 3 to 4 times the value of the gate-source capacitance of the transistor.
The resistor is used for adjusting the value of the DC source potential relative to ground. It is thus to have a sufficiently high value for a convenient, not negligible, potential to be established at its terminals.
This resistor-capacitor arrangement has a detrimental effect in that it creates or amplifies oscillations which already have the tendency of occurring anyhow in a high-gain distributed amplifier.
On the other hand, this arrangement is applied to transistors of the interdigitated type which have one drain finger, two gate fingers arranged on either one of the two sides of the drain finger and, futhermore, two source fingers arranged on either one of the two sides of the ensemble.
In an embodiment of prior-art circuit the transistors are disposed in line on a substrate, while the electrodes are arranged in parallel, the source fingers of one transistor thus being opposite to a source finger of the preceding transistor and a source finger of the next transistor, except for the first and last transistors.
A metal coating is disposed between the transistors to form a ground stub. This metal coating is connected to a ground plane realised on the opposite side of the substrate by a via hole.
In a particular embodiment of prior-art circuit illustrated by FIG. 15 of said document, each source finger is connected via an air bridge to a planar capacitor realised on the surface of the ground stub, so that in this manner two capacitors are realised facing each other between the transistors and being in line with these transistors. For each transistor one of the source fingers is lengthened by a conducting line to connect this source finger to a resistor located in a part of the substrate surface between the conducting line and the ground stub, but not in line with the transistors.
According to said document this configuration has been chosen to avoid modifying the dimensions of the microwave lines which form inductors that connect the drains of the transistors to form the drain line, as well as those of the microwave lines that have a similar function in forming the gate line.
The problem is actually that the capacitors of the self-bias circuits to be realised between source fingers and ground stubs have, as a result of their function, rather large dimensions; but because of their planar structure they can be realised on the surface of the ground stub and, using the shape of an isolation slit made between two facing capacitors, they can be integrated without causing the distance between transistors to be increased too much. In contrast, it is impossible in this embodiment also to place the resistors of the self-bias circuits in line with the transistors and the capacitors, for if they are inserted between the transistors this would entail too large changes of the dimensions of the gate line and drain line compared with a similar circuit not including source self-bias systems, due to the fact that the dimensions of these resistors are rather large.
In said described embodiment the two source fingers of the same transistor are not interconnected and only one of these fingers comprises the RC self-bias circuit. Thus, the transistors in FIG. 15 of said document operate each as a transistor half.
On the other hand, it will be noted that the resistors of the self-bias circuits are located exactly between the sources of the transistors and ground, because the inductances of the via holes, which holes connect the ground stubs to the ground plane, are negligible compared with the impedances which constitute these self-bias circuits and are thus not taken into account for the operation thereof.
The need to realise high-gain amplifiers which have a large bandwidth up to high or microwave frequencies, has provided that the distributed amplifier can be chosen as an element for this sort of application.
Microstrip technology is the technology selected for realising the distributed amplifier known from said document.
In this technology a first surface called the front surface of the substrate which is here made of GaAs carries the active elements, conducting strips to form transmission lines and metal islands to form the ground stubs. And the second surface of the substrate called rear surface of the substrate carries a metal island to form ground, so that the line impedance is a function of the width and length of the stripline conductors of the first surface and of the thickness of the substrate. Moreover, via holes located in the central parts of the ground stubs are formed through the substrate, to bring the potential of the ground plane on the rear surface of the substrate to these ground stubs on the front surface of the substrate.
Said document describes that when optimizing means are used for optimizing the distributed amplifier circuit to increase the width of the passband and/or increase the gain, there will be a drawback: the parasitic elements inherent in the circuit will generate oscillations.
Particularly at high or microwave frequencies the parasitic gate-drain capacitance (C.sub.gd) becomes a very critical parameter and oscillations appear at frequencies which are higher than that of the passband of the amplifier.
In the state of the art there are other distributed circuits such as mixers which are formed in stages, for example, of the cascaded type and which are subject to the same drawbacks as the distributed amplifiers.